The Two Families of Pumps
Almost every pump in existence belongs to one of two broad families: centrifugal pumps or positive displacement (PD) pumps. Understanding the fundamental difference between them is essential for anyone working with fluid systems.
How Centrifugal Pumps Work
A centrifugal pump uses a rotating impeller to impart kinetic energy to the fluid. As fluid enters the eye of the impeller, centrifugal force flings it outward into a volute casing, converting velocity into pressure. The result is a continuous, smooth flow.
Key characteristics:
- Flow rate varies significantly with system pressure (head)
- Best suited to low-viscosity fluids like water
- High flow rates at moderate pressures
- Cannot self-prime in most configurations
- Simple design, low maintenance, widely available
How Positive Displacement Pumps Work
A positive displacement pump traps a fixed volume of fluid and forces it through the discharge. Each cycle — whether from a rotating gear, a reciprocating piston, or a flexible diaphragm — delivers the same volume regardless of downstream pressure.
Key characteristics:
- Flow rate remains nearly constant regardless of pressure
- Handles viscous, shear-sensitive, or abrasive fluids well
- Capable of very high pressures
- Generally self-priming
- More complex, higher maintenance than centrifugal pumps
Side-by-Side Comparison
| Factor | Centrifugal | Positive Displacement |
|---|---|---|
| Flow consistency | Varies with pressure | Constant regardless of pressure |
| Viscosity handling | Low viscosity only | Low to very high viscosity |
| Pressure capability | Moderate | Very high |
| Self-priming | Usually no | Usually yes |
| Pulsation | Smooth, non-pulsating | Can pulsate (reciprocating types) |
| Typical cost | Lower | Higher |
| Common uses | Water supply, HVAC, irrigation | Dosing, hydraulics, food processing |
Subtypes of Positive Displacement Pumps
PD pumps themselves split into two groups:
Reciprocating PD Pumps
These use a piston, plunger, or diaphragm moving back and forth. Examples include piston pumps (high pressure, oil and gas), diaphragm pumps (chemical dosing, sanitary applications), and plunger pumps (pressure washing, hydraulic systems).
Rotary PD Pumps
These use rotating elements to move fluid. Examples include gear pumps (oil transfer, hydraulics), lobe pumps (food and pharmaceutical), screw pumps (high-viscosity fluids), and peristaltic pumps (laboratory, medical dosing).
Which Should You Choose?
Choose a centrifugal pump when you need high flow rates of low-viscosity fluids and a smooth, continuous flow at moderate pressure. Choose a positive displacement pump when you need precise flow control, must handle viscous or sensitive fluids, or require very high pressures with consistent output.
Many industrial installations use both types at different stages of the same process — each doing what it does best.